KABUL (Reuters) - At least 15 civilians were killed and 13 wounded in a U.S. unmanned aircraft strike in eastern Afghanistan, the United Nations said, calling for an independent investigation into the incident.

The airstrike early on Wednesday morning hit what U.S. officials said was an Islamic State target in the Achin district of Nangarhar province.

Government officials said the strike killed some militants, but the U.N. Assistance Mission in Afghanistan (UNAMA) reported the victims were civilians, including students, a teacher, and members of families considered to be "pro-government".

"UNAMA reiterates the need for all parties to the conflict to adhere to their obligations under international humanitarian law," the United Nations said in a statement late on Thursday.

"UNAMA calls on the government and international military forces to launch a prompt, independent, impartial, transparent, and effective investigation into this incident."

All the civilians reported killed were men, it added.

The men had gathered in a village to welcome a local elder on his return from completing the Hajj pilgrimage to Mecca and were sleeping when the strike happened, witnesses said.

The strike occurred in a remote area near the Pakistan border that has become a stronghold for Islamic State in Afghanistan.

The U.S. military command confirmed the air strike in Achin was aimed at Islamic State militants, but said it was gathering information on the allegations of civilian casualties.

"We take every possible measure to avoid civilian casualties in these operations, and will continue to work with Afghan authorities to determine if there is cause for additional investigation," U.S. forces said in a statement.

President Barack Obama this year authorized American forces to target Islamic State militants in Afghanistan, leading to regular strikes by both drones as well as manned warplanes.